1. Field of the Invention
The present invention relates to an improved angular contact ball bearing for use in a vehicle wheel.
2. Description of the Prior Art
Conventionally, for a vehicle wheel, especially, for a car wheel, there is often used a double row ball bearing. For example, in the case of such a bearing for a car wheel as shown in FIG. 1, although the rotation speed is relatively low, the weight of the car is applied directly to the bearing, so that the bearing receives a large radial load. Also, when a car takes a sudden turn, depending on an acceleration applied onto the car, further larger radial, axial and moment loads are applied to the bearing. Therefore, there has been employed a bearing which is as compact as possible in size but is as large as possible in the load capacity.
In order to reduce the bearing load of a bearing for a car wheel and thereby enhance the durability thereof, there has been employed a design in which a ball-to-ball distance (bearing-to-bearing distance) is set as long as possible to thereby make longer a distance between operation points. Therefore, the length L of the shoulder portion 3a of an inner race 3 shown in FIG. 2 is shortened. This tendency is outstanding especially in a bearing of a type that no seal is attached to the shoulder portion 3a.
In the bearing for a car wheel, in order to prevent the bearing from interfering a corner portion provided in the root of the shoulder portion of the inner race with a shaft which is supported by the bearing, there is formed a relatively large escape portion 3c.
Referring to a roller bearing, if it is mounted on a shaft with a small interference and is rotated with a load applied to the inner race thereof, there is a possibility that there can occur a harmful slippage in the circumferential direction thereof between the inner race and shaft. This slipping phenomenon of the inner race is generally referred to as a creep. In particular, when the fitting surface of the inner race has a short interference, the point of loading is moved in the circumferential direction to thereby shift the position of the inner race in the circumferential direction with respect to the shaft or housing.
In a bearing for a car wheel of a type that an inner race thereof can be rotated, there can be produced a creep phenomenon that the inner race and shaft are shifted in position in the rotation direction. When such creep phenomenon occurs, then the fitting surfaces of the inner race and shaft slip with respect to each other so that the fitting surfaces thereof are worn and damaged. As a result of this, the fitting engagement between the inner race and the shaft is loosened to thereby produce a strange sound. And, if the inner race is damaged heavily, then a service life of the bearing itself is possibly shortened, the inner race is possibly broken, and the inner race and the shaft may score each other.
As a countermeasure against the creep phenomenon, it is expected that a large interference is set between the inner race and the shaft and a axial tension for holding the inner race in the axial direction is increased. On the other hand, there is a limit to increase such interference, because a tensile stress acting in the circumferential direction is applied to the inner race of the bearing. In addition, if the interference is increased, then it becomes more difficult to mount the shaft. Further, when the axial tension is increased, the screw portion and nut portion that are used to apply such axial tension must be checked for the rigidity thereof and, at the same time, it becomes more difficult to control the axial tension.